The present invention relates to an air-to-fuel ratio control system for an internal combustion engine. More specifically, the invention relates to an electric-type air-to-fuel ratio control system for an internal combustion engine which is equipped with a device for electrically controlling the supply of air to the engine during times of deceleration.
In one prior art air-to-fuel ratio control system, each cylinder is provided with an injection valve which is electrically driven in accordance with the flow rate of intake air to the engine. In such a system, however, the operating condition of the engine can easily be adversely affected due to differences in the air-to-fuel ratio among various ones of these cylinders. Differences in the air-to-fuel ratio among the cylinders can also cause the constituency of the exhaust gas to fall outside of allowable air pollution standards. Moreover, if a negative feedback control system is used to control the driving (open) times of the fuel injection valve provided for each cylinder, unless separate air-to-fuel ratio detectors are provided for each cylinder at positions upstream of the merging point of the exhaust manifold, it is impossible to effect the feedback control with a high degree of precision. Also, in order to maintain the air-to-fuel ratio as constant as possible among the cylinders, the fuel injection valves must as nearly as possible be identical in injection characteristics. To construct fuel injection valves having the requisite highly accurate characteristics is expensive.
In another type of known air-to-fuel ratio control system, a single injection valve is provided upstream of the branching point of the intake manifold of the engine in order to eliminate the above described drawbacks accompanying the multi-injection valve type control system. In this type of system, the relatively long distance between the fuel injection valve and the intake valves of the cylinders causes a delay from the time that fuel is injected until it arrives in the cylinders due to the fact that the injected fuel is in a liquid state and hence moves at a slower speed than the intake air. Because of this delay, the air-to-fuel ratio of the intake mixture into the cylinders may be far from what is desired when the speed of the engine is abruptly changed. For instance, if the accelerator is quickly released, the delay between the injection of the fuel and the time that it arrives in the cylinders causes the mixture arriving in the cylinders to be overly rich for a significant amount of time after the accelerator peddle is released. This leads to backfiring of the engine and abnormal shocks on the engine caused by excess amounts of fuel in the cylinders. Also, some of this excess fuel may not be burned so that hydrocarbons may be discharged in high concentrations in the exhaust gas.
It is a primary object of the present invention to eliminate the aforementioned problems inherent with the prior art air-to-fuel ratio control systems.